This invention concerns methods and arrangements for liquid cooling of structures contacting very hot materials which prevent the development of excessively high temperatures in the structure which can cause mechanical failures due to thermal stress. In conventional liquid cooling, liquid coolant typically flows through vessels in contact with the structures and a loss of cooling capacity may occur if the liquid coolant flowing in coolant vessels associated with the structures boils. This is a particular problem in conveyors such as auger or re-circulating chain flight conveyors used to convey very hot crushed or granular material exceeding 1000° F. through troughs such as in cement plants, lime kilns, power plants, etc.
Conveyors for such very hot materials have in the past had short service lives and were prone to failure. This is because of the effect of the high temperatures reached by the conveyor components as a result of conduction of heat from the conveyed material into the structure and components. Such conveyors have sometimes incorporated liquid cooling jackets within the conveyor trough along which the hot material is conveyed as by an auger extending along the length of the trough. In the past, the trough and jacket have been constructed as a weldment, and since the liquid cooled liner is in direct contact with the hot material conveyed, the welds are severely stressed by gross thermal expansions and contractions.
The resulting expansion and contraction of the trough and coolant jacket leads to cracking, buckling, weld failures and similar structural failures. If very hot material is conveyed (1000° F. or higher), cooling liquid in direct contact with the cooling jacket wall is heated to boiling, so that vapor is generated in the jacket, greatly reducing the rate of heat conduction into the cooling liquid.
The high heat flux boiling that is encountered, usually has regions of unstable film boiling which causes a thermal shock in the structure surface, which in turn can cause plastic mechanical behavior. This can lead to premature failure and has been studied mathematically and experimentally. See Kappila, R. W., “A Boiler Tube Problem, Elastic-Plastic Behavior of a Thick-Walled Cylinder Caused By Sinusoidal Inside Surface Temperature, Internal Heat Generation, and External Heat Flux,” PhD Dissertation, University of Michigan, 1968.
Since the trough cooling jacket is constructed as a weldment, it often is not designed or approved for use as a pressure vessel, allowing only very low coolant pressures and thus low flow rates imposing a substantial limitation on the rate of heat removal.
Similarly, conveying augers have also often been constructed as a weldment, with a central tube having radial spokes welded to a central tube forming a triangular cavity. Liquid coolant has sometimes been circulated through such an auger, with direct contact of the coolant with the metal auger which in turn is in direct contact with the hot material conveyed, leading to the same problems described above in connection with the conveyor trough.
Direct air cooling of the hot material requires dust collection equipment and baghouses and necessitates government permits, as pollutants may be mixed with the exhausted cooling air.
Many other industrial applications and high technology projects experience such difficulties, such as, screw conveyors in hot quick lime production, power plant hot clinker removal, hot surfaces of space vehicles during re-entry into the earth's atmosphere, cooling high temperature engines and jets, boilers, etc.
It is an object of the present invention to provide arrangements and methods to control heat transfer into a liquid coolant within a flow vessel used to cool a hot material of the type described, in which direct contact of a liquid coolant with the structure holding the hot material is avoided.
It is a further object to provide a conveyor for hot material which avoids the use of weldments to mount parts subjected to thermal stresses induced by a large temperature differential between connected parts of the conveyor.